With high integration and large capacity of a Large Scale Integration (LSI), a circuit dimension required for a semiconductor element becomes increasingly narrow. Using an original image pattern (that is, a mask or a reticle, hereinafter collectively referred to as a mask), a reduced-projection exposure apparatus called a stepper or a scanner exposes and transfers the pattern on a wafer to form a circuit, thereby producing the semiconductor element.
It is necessary to improve a production yield for costly LSI production. At this point, a shape defect of the mask pattern can be cited as a large factor that degrades the production yield.
On the other hand, there is a demand for pattern formation having a line width of tens nanometers in a contemporary typical logic device. The shape defect of the mask pattern also becomes finer in such a situation. Because dimension accuracy of the mask is enhanced in order to absorb fluctuations of various process conditions, it is necessary to detect the defect of the extremely small pattern in mask inspection. Therefore, high accuracy is required for an apparatus that evaluates the pattern of a transfer mask used in the LSI production. For example, Japanese Patent No. 4236825 discloses an inspection apparatus that can detect the fine defect on the mask.
Recently, as a technique for forming a fine pattern, nanoimprint lithography (NIL) has attracted attention. In this technique, a template having a nanoscale microstructure is pressured on a specific resist formed on a wafer to form the fine circuit pattern on the resist.
In nanoimprint technology, in order to increase productivity, plural duplicate templates (replica templates) are produced using a master template, that is, of an original plate, and the replica templates are used while attached to different nanoimprint apparatuses. It is necessary that the replica template be produced so as to correspond precisely to the master template. Therefore, it is necessary that not only the pattern of the master template but also the pattern of the replica template be evaluated with high accuracy.
Generally the mask is formed with a dimension four times the circuit dimension. The pattern is reduced and exposed onto a resist on the wafer by a reduced projection exposure device, using the photo-mask, and thereafter, the circuit pattern is developed. On the other hand, in the nanoimprint lithography, the template is formed with magnification equal to the circuit dimension. Therefore, the shape defect in the pattern of the template has the large influence on the pattern transferred onto the wafer compared with the shape defect in the pattern of the mask. Accordingly, it is necessary to evaluate the pattern of the template with higher accuracy compared with the case that pattern of the mask is evaluated.
Recently, with the progress of the fine circuit patterns, the pattern dimension is finer than a resolution of an optical system of a pattern evaluation apparatus. For example, when a line width of the pattern formed in the template is less than 50 nm, the pattern cannot be resolved by a light source of DUV (Deep UltraViolet radiation) light having a wavelength of about 190 nm to about 200 nm. The optical system is relatively easily constructed for the DUV light having the wavelength of about 190 nm to about 200 nm. Therefore, the light source of an EB (Electron Beam) is used. However, unfortunately the light source of the EB is not suitable for quantity production because of low throughput.
There is a demand for an inspection apparatus that can accurately inspect the fine pattern without generating the throughput degradation.
There are various types of defect in the pattern. Among others, a short-circuit defect in which the lines short-circuit to each other and an open-circuit defect in which the line is disconnected have the largest influence on performance of the mask or template. FIG. 1 illustrates an example of the short-circuit defect. The two adjacent lines are connected to each other in a region A1 to generate the short-circuit defect. FIG. 2 illustrates an example of the open-circuit defect. The line is partially disconnected in a region A2.
On the other hand, for the defect in which the edge roughness increases as seen in a region A3 in FIG. 3, the defect has a restrictive influence on the performance of the mask or template.
Even though all defects can be detected, that is, defects that may cause a problem and defects that will not cause a problem, the inspection can be efficiently performed when only the defect that may cause a problem is detected. However, the short-circuit defect, the open-circuit defect, and edge roughness (shown in the region A3 in FIG. 3) are less than or equal to a resolution limit. In the case that the short-circuit defect, the open-circuit defect, and the edge roughness are mixed in a repetitive pattern having a period at the resolution limit or less, brightness and darkness caused by the defect, such as the short-circuit defect and the open-circuit defect, which becomes a problem, and brightness and darkness caused by the edge roughness are not distinguished from each other in observation with the optical system. The same holds true for a bright-field image and a dark-field image. This is because the short-circuit defect, the open-circuit defect, and the edge roughness are identical in size, namely, spread to the size of an extent of the resolution limit in an optical image.
FIG. 4 schematically illustrates a line and space pattern. In FIG. 4, it is assumed that the pattern dimension is smaller than the resolution limit of the optical system. In a region B1 of FIG. 4, the line pattern is partially lacks. In a region B2, the pattern edge roughness increases. The defects are clearly distinguished from each other on an actual substrate. However, the defects cannot be distinguished from each other when observed through the optical system. This is because the optical system acts as a spatial frequency filter that is defined by a wavelength λ of the light emitted from the light source and a numerical aperture NA. FIG. 5 illustrates an example in which the spatial frequency filter is applied to the pattern in FIG. 4. The defect in the region B1 and the defect in the region B2 are identical in size, and the difference of the shape is hardly recognized. Accordingly, the short-circuit defect which is less that the resolution limit is difficult to be distinguished with the defect caused by edge roughness which is also less than the resolution limit.
The present invention has been devised to solve the above problems. An object of the present invention is to provide an inspection apparatus that can accurately inspect the fine pattern without generating the throughput degradation, more particularly an inspection apparatus that can distinguish the defect to be detected from a defect that is not to be detected.
Other challenges and advantages of the present invention are apparent from the following description.